gistfile1.scala

  trait HListContains[-H <: HList, A] { def apply(h: H):A }

  implicit def headContains[A, T <: HList] = new HListContains[A :: T, A] {
    def apply(h: A :: T) = h.head
  }

  implicit def tailContains[A, H, T <: HList](implicit tc: HListContains[T, A]) = new HListContains[H :: T, A] {
    def apply(h: H :: T) = tc(h.tail)
  }